1. Field of the Invention
The present invention relates to a zirconium oxide (ZrO.sub.2) sinter suitable for forming a thin film thereof and the method for production of the same. More specifically, the invention relates to a high purity zirconium oxide sinter suitable for use in, for example, vacuum evaporation coating and sputtering and relates to the method for production of the same.
The zirconium oxide (ZrO.sub.2) thin film produced by vacuum evaporation coating or sputtering is applicable widely in optical uses because of its excellent properties, such as transparency, high hardness, high refractive index and high environmental resistance.
2. Description of the Related Art
Since ZrO.sub.2 is subject to a phase transformation (monoclinic.revreaction.tetragonal) accompanying a large change in volume in a temperature range from 900.degree.-1200 .degree. C., it has conventionally been difficult to produce a high density sinter of pure ZrO.sub.2. That is to say, for producing ZrO.sub.2 sinter in the past, 3 mol % or more of a stabilizer such as CaO, MgO or Y.sub.2 O.sub.3 was added for the purpose of stabilizing ZrO.sub.2 crystals in the tetragonal form, and the crystals were sintered at a temperature not lower than 1400 .degree. C. so as to obtain stabilized (or partially stabilized) ZrO.sub.2 sinter. However, these sinters contain unavoidably a relatively large quantity of the stabilizer, which lowers the purity, and therefore, the zirconium oxide film obtained therefrom by vacuum evaporation coating or sputtering is not satisfactory for use as an optical film because of low transparency and refractive index, in particular.
Several proposals have been presented for producing a ZrO.sub.2 sinter having a density corresponding nearly to the true density of high purity ZrO.sub.2 (monoclinic, density by X-ray: 5.83 g/cm.sup.3), as disclosed by A. C. D. Chaklader and V. T. Baker, Am. Ceram. Soc. Bull., 44, 258-59 (1965); G. K. Bansal and A. H. Heuler, J. Am. Ceram. Soc. Bull., 58, 76-77 (1979); H. J. Garrett and R. Ruh, Am. Ceram. Soc. Bull., 47, 578-79 (1968); M. Yoshimura and S. Somiya, Am. Ceram. Soc. Bull., 59, 246 (1980); Murase, Kato and Hirano, J. Ceram. Soc., Japan, 91, 561-64 (1983). However, all of the proposed methods involve complicated processes such as the use of a hot press, reaction hot press, reoxidation after vacuum sintering, hydrothermal synthesis from Zr and water, and the use of superfine particles obtained by hydrolysis of ZrOCl.sub.2, and therefore the production cost of the sinter by these methods is unavoidably very high. Moreover, the sinter products obtained according to these methods suffer residual stress which makes them very susceptible to cracking in the course of their production process, and consequently, a large size sinter suitable for producing a thin film thereof cannot be obtained, and further the sinter products thus obtained very easily crack when they are used as the source for vacuum evaporation coating and the target for sputtering. Accordingly, it has been a common practice to use a sinter product with a density of not more than 4.9 g/cm.sup.3 (or generally 4.0-4.5 g/cm.sup.3) for producing a high purity ZrO.sub.2 film. However, in using such a sinter product as an evaporation source for radiation-type vacuum evaporation coating with the use of an energy beam such as an electron beam (hereinafter will be called in short as EB), there will be such a defect that a uniform film can hardly be obtained due to the formation of large recesses on the surface of the evaporation source where the beam is directly radiated when the beam power is increased, and further in the case of applying such a sinter product as the evaporation source in a resistance heating-type vacuum coating process and the target in the sputtering process a uniform film can also hardly be obtained due to the influence of a large amount of gas absorbed in the low-density sinter. Moreover, the sinter products obtained by the prior art have only a short life as the source for the formation of a thin film thereof, and are thus very low in productivity.